Update dependency protobuf to v3.15.0 [SECURITY]

requirements.txt

@@ -16,7 +16,7 @@ idna==2.10; python_version >= "2.7" and python_full_version < "3.0.0" or python_
 loguru==0.5.3; python_version >= "3.5"
 packaging==20.9; python_version >= "3.6" and python_full_version < "3.0.0" or python_full_version >= "3.6.0" and python_version >= "3.6"
 proto-plus==1.13.0; python_version >= "3.6"
-protobuf==3.14.0; python_version >= "3.6" and python_full_version < "3.0.0" or python_full_version >= "3.6.0" and python_version >= "3.6"
+protobuf==3.15.0; python_version >= "3.6" and python_full_version < "3.0.0" or python_full_version >= "3.6.0" and python_version >= "3.6"
 pyasn1-modules==0.2.8; python_version >= "2.7" and python_full_version < "3.0.0" or python_full_version >= "3.6.0"
 pyasn1==0.4.8; python_version >= "3.5" and python_full_version < "3.0.0" and python_version < "4" and (python_version >= "3.6" and python_full_version < "3.0.0" or python_full_version >= "3.6.0" and python_version >= "3.6") or python_version >= "3.5" and python_version < "4" and (python_version >= "3.6" and python_full_version < "3.0.0" or python_full_version >= "3.6.0" and python_version >= "3.6") and python_full_version >= "3.6.0"
 pycparser==2.20; python_version >= "3.6" and python_full_version < "3.0.0" or python_full_version >= "3.6.0" and python_version >= "3.6"